There is known in the art a mobile terminal device having a function of automatically changing a direction of an image displayed on a screen of the mobile terminal device in response to inclination of the mobile terminal device. (Hereinafter, this function is referred to as a “display rotation function.”)
FIG. 1 shows how an image is displayed by mobile terminal device 9 having the display rotation function known in the art. The downward direction in FIG. 1 corresponds to a vertical downward direction in the real world. In the following explanation, a state of mobile terminal device 9 with a long side of its rectangular shaped screen extending in a vertical direction as shown in FIG. 1(a) is referred to as a “vertically long state,” and a state of mobile terminal device 9 with a short side of its screen extending in a vertical direction as shown in FIG. 1(b) is referred to as a “horizontally long state.” Moreover, in the following explanation, a position or a direction of a screen is specified by a combination of an x-coordinate and a y-coordinate in a coordination system in which the origin is the left lower apex of the screen in the vertically long state; the positive direction of a y-axis is the upward direction in the vertically long state; and the positive direction of an x-axis is the rightward direction in the vertically long state.
When the display rotation function is in operation, as shown in FIGS. 1(a) and 1(b), mobile terminal device 9 displays an image in a state where the vertical direction of a content of an image fits the vertical direction of the real world in both the vertically long state and the horizontally long state. Namely, when mobile terminal device 9 is in the vertically long state as shown in FIG. 1(a), mobile terminal device 9 displays the image so that the vertical direction of the content of the image fits the y-axis, and when mobile terminal device 9 is in the horizontally long state as shown in FIG. 1(b), mobile terminal device 9 displays the image so that the vertical direction of the content of the image fits the x-axis. Accordingly, when mobile terminal device 9 is rotated, for example, in a counterclockwise direction by 90 degrees and the state of mobile terminal device 9 changes from the vertically long state as shown in FIG. 1(a) to the horizontally long state as shown in FIG. 1(b), mobile terminal device 9 rotates the displayed image on the screen in a clockwise direction by 90 degrees.
A user of mobile terminal device 9 having the display rotation function is able to see the image on the screen as an upright image both when the user holds mobile terminal device 9 in the vertically long state and when the user holds mobile terminal device 9 in the horizontally long state.
JP2003-216135A is a document that discloses a technique of adjusting a direction of an image displayed on a screen in response to an inclination of a mobile terminal device. In the information display device disclosed in JP2003-216135A, a degree of inclination of the information displaying device is determined, and when information including both text and figures is displayed on the screen of the device, for example, only text is rotated in response to a determined degree of inclination of the device, thereby to display the text as an upright image.
In a mobile terminal device with the known display rotation function, a direction of a displayed image may not always fit a viewing direction that a user of the mobile terminal device wishes to use. (In this application, an “image” shown on a screen may contain any sort of information such as figures, text, symbols, and so on.) For example, when the user holds the mobile terminal device in a state where a screen of the mobile terminal device is generally parallel with a horizontal plane, a direction of an image displayed on the screen of the mobile terminal device may, contrary to the user's intention, be rotated by the display rotation function in response to a small change in inclination along the x-axis direction or the y-axis direction. For example, when the user is lying on the floor and views the screen of the mobile terminal device, since the horizontal direction becomes an upright direction of the image for the user, the image displayed by the terminal device rotated by the display rotation function becomes prone from a view point of the user and thus cannot be viewed with comfort by the user.